1. Field of the Invention
This invention relates to an internal step-down power supply circuit of a semiconductor device. More particularly, it relates to the construction of a power supply circuit using a fuse circuit for regulating a potential of an internal step-down voltage power supply voltage.
A requirement for lower power consumption has become stronger and stronger in semiconductor devices, and a circuit technology for bringing a DC path current occurring in a fuse circuit to as close to zero as possible has become necessary. Nonetheless, existing power supply circuits cannot entirely satisfy such a requirement, and improvements must be yet made. Therefore, a circuit technology for satisfying the requirement is necessary.
2. Description of the Related Art
It is generally a common practice in semiconductor devices to lower an external power supply voltage to a power source voltage required inside a chip and to use the voltage (so-called "internal step-down voltage") so as to reduce power consumption, to improve a withstand voltage of an oxide film, to keep a power supply voltage constant, and so forth.
FIG. 1 shows an example of characteristics of an internal step-down power supply voltage (VII) relative to an external power supply voltage (VCC). The example shown in the drawing represents the case where a product using VCC=3.3 V as the external power supply voltage is used.
A recommended operation range of products using VCC 3.3 V is generally from 3.0 to 3.6 V according to their specifications. However, an internal step-down voltage power supply circuit, which is controlled so as to become flat (or constant) at VII=2.4 V is used in practice inside the chip in order to accomplish the objects described above.
On the other hand, the occurrence of detective products is unavoidable in semiconductor devices. The forms of such product defects are dominantly the type in which defects appear immediately after the production and the type in which the defects appear in the course of time due to life of the products, as is well known from a so-called "bathtub curve". The former is referred to as "initial defect".
Accelerated tests such as a burn-in (B.I.) test have been conducted in the semiconductor devices to eliminate the initial defect at an early stage. This test is carried out by imposing a severe condition (for example, a condition under which the devices-are operated for a long time at a voltage higher than the recommended operation range and at a high temperature) on the devices.
In the example shown in FIG. 1, a range of VCC.gtoreq.4 V is set to the accelerated test range. Inside this range, the internal step-down power supply voltage (VII) is released from the flat state of VII=2.4 V and is controlled in such a manner as to follow the external power supply voltage (VCC) and to attain a high potential.
For the reasons described above, the heretofore known internal step-down power supply circuits generally comprise a power supply voltage control portion for a normal operation and a power supply voltage control portion for a test operation. FIG. 2 shows one structural example.
In FIG. 2, reference numeral 50 denotes a VII potential regulation circuit for a normal operation. This circuit 50 includes a fuse circuit 51 using fuse elements for making it possible to regulate a potential of an internal step-down voltage VII in accordance with a potential of a node N after completion of the process, and a decoder circuit 52 for decoding information representing a cut-off state of the fuse elements in the fuse circuit 51. Reference numeral 53 denotes a VII potential control circuit for a normal operation which controls the potential of the node N on the basis of the decoding result of the decoder circuit 52, reference numeral 54 denotes a VII generation circuit for a normal operation which generates an internal step-down power supply voltage in response to the output (potential of the node N) of the VII potential control circuit 53 for the normal operation.
Similarly, reference numeral 55 denotes a VII potential regulation circuit for a test operation. This circuit 55 includes a fuse circuit 56 using fuse elements for making it possible to regulate the potential of the internal step-down power supply voltage VII in accordance with the potential of the node N after process-out, and a decoder circuit 57 for decoding information representing the cut-off state of the fuse elements in the fuse circuit 56. Reference numeral 58 denotes a VII potential control circuit for the test operation for controlling the potential of the node N on the basis of the decoding result of the decoder circuit 57. The output of this VII potential control circuit 58 for the test operation is connected to the node N. Therefore, the VII generation circuit 54 generates the internal step-down power supply voltage VII in response to the output (potential of the node N) of the VII potential control circuit 58 in the test operation.
External power source voltages (VCC) are used as circuit power supplies of the VII potential regulation circuit 50 and the VII potential control circuit 53 for the normal operation, of the VII potential regulation circuit 55 and the VII potential control circuit for the test operation, and of the VII generation circuit 54, respectively.
In the circuit construction described above, the potential at the node N is decided by the output of each of the VII potential control circuits 53 and 58 for the normal and test operations. When products are manufactured under undesirable conditions such as a process fluctuation, however, the voltage for controlling the VII potential (potential at the node N) at a target cannot be outputted in most cases.
Therefore, in order to make it possible to regulate this voltage for controlling the VII potential (potential of the node N) after completion of the process, the VII potential regulation circuit 50 for the normal operation and the VII potential regulation circuit 55 for the normal operation are provided.
In other words, each fuse element in the fuse circuit 51 inside the VII potential regulation circuit 50 is kept appropriately in the cut-off state in the normal operation, information representing the cut-off state of this fuse element is decoded by the decoder circuit 52, and the potential of the node N is regulated by the VII potential control circuit 53 on the basis of this decoding result.
Similarly, several fuse elements in the fuse circuit 56 inside the VII potential regulation circuit 55 are kept under the cut-off state in the test operation, information representing the cut-off state of these fuse elements is decoded by the decoder circuit 57, and the potential of the node N can thus be regulated by the VII potential control circuit 58 on the basis of this decoding result.
As described above, the prior art technology uses the fuse circuits 51 and 56 for making it possible to regulate the potential of the node N (that is, the potential of the internal step-down power supply voltage VII) after completion of the process. Therefore, a DC path current occurs through the fuse elements in each fuse circuit. As attempts have been made to further decrease power consumption at present, the value of this DC path current becomes a value that can never be neglected in comparison with the current value allowed by product specifications.
Further, the prior art technology uses the external power supply voltages (VCC) for all the circuits that constitute the internal step-down power supply circuit and for this reason, the problem described above appears all the more remarkable in some cases. In other words, if the cut-off state of each fuse element in the fuse circuit is not complete (or if the resistance value is not very high) in-this circuit construction, the DC path current increases proportionally, As a result, the greater the number of the fuse elements, the more remarkable becomes this problem. A consumed current value of .mu.A order exists in the latest device catalogues, and lower power consumption must be by all means accomplished. Since the DC path current in this fuse circuit is likely to govern the specification value of a .mu.A order, the current value must be reduced to a minimum.
When the DC path current increases, the voltage appearing across both ends of each fuse element (that is, information representing the cut-off state of the fuse element) is likely to become unstable, so that operational reliability of the fuse circuit itself might fall. This makes the operation of the internal step-down power supply circuit unstable as a whole.